In a conventional CMOS image sensor, photodiodes that are photoelectric converters to form pixels are formed in an array on a first face of a semiconductor substrate on the light incident side, and transistors for reading the charges converted by the photodiodes, wiring layers, and the like are also formed on the same first face above the photodiodes. Therefore, the aperture size relative to the light incident faces of the photodiodes is limited by the existence of the wiring layers and the transistors, and a 100% aperture ratio cannot be obtained. As a result, the incident light use efficiency has been poor.
To counter this problem, a technique of applying a structure of a back-illuminated type in which light is incident on a second face on the opposite side from the wiring layers to CMOS image sensors has been recently suggested as a technique for dramatically increasing aperture ratios. In a structure of a back-illuminated type, the wiring layers, the transistors for reading, and the like are formed on the first face of a semiconductor substrate, and a photodiode array that photoelectrically converts light into signals is formed on the second face to be the light-illuminated face. Color filters and the like for dividing incident light into wavelength regions, such as R (red), G (green), and B (blue) are formed on the light-illuminated face, and microlenses for gathering light are further formed on the color filters.
In the conventional structure of the back-illuminated type, however, it is difficult to prevent electrical color mixing that occurs due to movement of photoelectrically-converted charges to adjacent pixels, and increase the light use efficiency at the same time. To prevent electrical color mixing, a technique such as separation by potentials using pn junctions through ion implantation is used, for example. However, if high-density impurity ions are implanted to strengthen the pixel separation by potentials in a pixel array having a pixel pitch narrower than 2 μm, the pixel separating regions become wider, and the sensitivity becomes lower in short-wavelength regions.
In view of this, as suggested in the back-illuminated type, for example, a trench separator in which a trench that is often used as a device separator in miniaturized LSIs is filled with an oxide film is used in each pixel separating region, so as to prevent electrical color mixing. In a case where such trench separators are used for pixel separation in a solid-state imaging device, however, the dark current generated from the interface states between Si and SiO2 existing in the sidewalls of the trench separators increases noise components.
Also, since the trench separators are formed on the wiring side, each of the trench separators has a tapered shape having a wider aperture on the wiring side. Therefore, if the trench separators are used in a solid-state imaging device of a back-illuminated type, each of the trench separators has a tapered shape with a narrower trench aperture on the light incident face. As a result, incident light passes through the photodiodes, and easily reaches the trench portions, causing the problem of optical color mixing.